Current metal processing methods use energy point sources to fabricate and repair metal parts. Such metal processing techniques include welding and additive metal layering. These processes use an energy point source such as a laser, electron beam emitter or plasma torch, to focus emitted heat energy to a workpiece. In all of these techniques an energy point source directs energy upon a workpiece and creates a melt pool where the focused energy is incident upon the workpiece. In the case of welding, the melt pool is often referred to as a “weld pool.” Additive metal deposition is one industrial process that uses focused emitted energy to build fully-dense structures by melting powdered or wire metal, via a laser or other energy source, into solidifying beads. The beads are deposited side by side and layer upon layer upon a workpiece substrate. It is known to utilize the process to repair and rebuild a worn or damaged component using a laser to build up structure on the component. The process is particularly useful to add features such as bosses or flanges on subcomponents of fabricated structures. The basic process involves adding layers to the component to create a surface feature on the component via the introduction of depositing material (delivered in the form of injected powder or a wire) into a laser beam. The additive process is known by several names including “laser cladding,” “laser metal deposition,” “direct metal deposition” or “additive metal layering.”
Additive metal layering is typically performed by using a computer aided design (“CAD”) to map the geometry of a part and then depositing metal on the part. The CAD mapped geometry is input into a computer-controlled (robotic) part handler that can manipulate the part in multiple axes of movement during the deposition process. In practice, the energy point source is under computer numerical control and emits focused energy onto a workpiece, producing the melt pool. A small amount of powder or wire metal is introduced into the melt pool, building up the part in a thin layer. The beam follows a previously determined toolpath. The toolpath is generated based on the CAD data that computes the needed part layer by layer. The beads are created by means of relative motion of the melt pool and the substrate, e.g. using an industrial robot arm or an XY-table, vis-à-vis the energy point source (beam source). (As used herein the action of moving the energy point source relative to the substrate means that either or both of the substrate or energy point source are moved to achieve relative movement.) The most popular approach combines a high power laser heat source with metal powder as the additive material.
The laser energy directed at the substrate (input energy) is absorbed by the substrate and causes local heat. The quality of the build is highly dependent on defining and maintaining the optimal conditions of the beam emitted by the energy point source. The objective is to accurately heat a desired volume of mass to achieve proper deposition of material. Temperature control is essential to achieve a successful build. Careful tuning of the deposition tool and parameters, such as the powder or wire feed rate, the energy input, and the traverse speed are therefore important in order to obtain layers that are free from defects such as shape irregularities, lack-of-fusion or cracks. Input energy may be modulated by changing the power level or duty cycle of the source. Droplet forming, i.e. globular transfer of the molten metal, is also a common disturbance that affects the geometrical profile of the deposited beads and stability of the additive layers.
Regulating the necessary needed input energy is critical to system operation and achieving a high-quality layered end product. The currently known laser additive processes attempt to address deposition quality issues in either of two ways. In this respect, the prior laser additive processes use a constant laser power or one regulated by a feedback (a/k/a “closed-loop”) sensor. The issue with using a constant laser power is that the operator has to optimize the power level for a worst case scenario, typically, the start of the process. This results in variations in both geometry and material properties as the melt pool size and temperature gradients vary with the local energy balance conditions around the melt pool. Using a constant energy throughout in an additive deposition process is problematic because the additive process changes the geometry of the built structure during the process. Hence, the chosen constant power level represents a compromise selection. For example and as shown in FIG. 1A, at the start of the deposition process, the structure is positioned further from the laser source and too little energy is input into the deposition. At the mid-process point, shown in FIG. 1B, the target structure is closer to the energy point source and the appropriate energy is present in the workpiece. However, by the end of the process, as shown, in FIG. 1C, the workpiece is closer to the energy point source and too much energy is present in the work site.
Feedback systems represent an attempt to address the deficits of the constant power system. In an attempt to improve welding and deposition processes, the prior art feedback systems concentrate their efforts on monitoring the intra-process dimensions or thermal characteristics of the melt pool to determine its area. Such systems use a camera or sensor to obtain an image of the melt pool region and then process the image to determine the size or shape of the melt pool. The shape of the melt pool is then used as an input to determine needed system action, e.g., increase or decrease laser power, increase or decrease beam dwell time, etc.
The prior art systems focusing on the melt pool area do not accurately account for the full, three-dimensional nature of the process (melt pool depth is not considered). Without considering depth, it is not possible to make an accurate estimate of the energy balance at the melt pool without making some assumptions. This is because material properties of metals processed via point heat source methods are highly correlated to the temperature gradients around the solidification front. There are other deficits of the prior art system that relies upon melt pool characteristics for feedback purposes. In this respect, the image processing required to find and measure the melt pool area greatly reduces the rate at which these measurements can be taken. The longer the measurement takes, the less useful it is for system control. Moreover, when imparting point source heat energy near an edge or corner, the shape of the melt pool will change its shape from that of a more interior location. Hence, feedback systems and methods that rely upon melt pool shape and emissions can give an incomplete and inaccurate assessment of the energy in the work environment. There is thus a need in the art for an improved method of regulating energy from an energy point source during metal processing techniques.